There is known a stirling engine for recovering exhaust heat. In this regard, for example, Patent Document 1 discloses a technique of the stirling engine itself. Also, for example, Patent Documents 2 and 3 disclose techniques to cause a common cooling medium to flow through an internal combustion engine and the stirling engine. Further, for example, Patent Document 4 discloses a technique to control output of the stirling engine for recovering the exhaust heat of the internal combustion engine.